Patent GB-1347993-A (GB)
Title: Sound recording and reporduction
IPC: H04H20/88
Filed: 1971-06-08
Abstract: 1347993 Stereophonic systems COLUMBIA BROADCASTING SYSTEM Inc 8 June 1971 [8 June 1970] 19487/71 Heading H4R In a stereophonic system additional channels of information are recorded on the normal two tracks by adding components of two additional information channels to each of the tracks. The components of one of the additional information channels are added to the tracks with the information on one track leading the information on the other, while the components of the other of the information channels are added to the tracks with the information on the one track lagging the information on the other track. As described the tracks can be recorded on a magnetic tape or a conventional stereophonic disc with the left front information L f , Fig. 7, given a phase shift #+45, where # is a reference value, and fed only to an adding circuit 60 providing one recording track L T . Left back information L b  is fed to the same adding circuit 60 with a phase shift #+0 and to the adding circuit 62, providing the other recording track R T , with a phase shift of #+90 degrees. The right front information R f  and left front information L f  are fed in corresponding fashion to the adding circuits 62 and 60 to provide the four information signals on the two tracks, the phase shifting circuits and adding of the components of L b  and R b  to both tracks resulting in a helical modulation of the groove walls, clockwise for one signal and counterclockwise for the other, while L f  and R f  provide conventional groove wall deviations. A centre signal C may be added to the original information signals either to both front channels, or both back channels or to all four channels. Vector diagrams in Fig. 7, indicate the relative phase shift given to the signal components by the phase shift networks 48 to 58. A decoding arrangement is shown in Fig. 9 in which the two composite signals are given an initial relative phase displacement of 90 degrees to provide for elimination or reduction of some of the component signals when the composite signals are added and subtracted in 90 and 98. Further phase shift networks 78, 92, 100, and 84 provide appropriate phase relationships at the reproducers 82, 88, 96, 104. A control circuit 106 can be provided to adjust the gains of amplifiers in the reproducer channels, as described in Specification 1,347,994. An alternative arrangement, Fig. 11 (not shown), dispenses with the combining circuits by using a transformer (130) to add the two composite signals together, and connecting a loudspeaker between the composite signal terminals to reproduce the difference signal. Alternatively the sum signal can be reproduced on a loud-speaker (160), Fig. 12 (not shown), connected in the common earth return lead of the left and right channel loudspeakers. Since this produces cross talk on the left and right channel speakers a resistive matrix (144, 146, 148) is provided to inject into those speakers a corresponding signal of opposite phase. In another arrangement, Fig. 13, a single speaker 162 is connected between the two composite signal channels to reproduce all the low frequency signals in those channels, an inverting amplifier 142 being provided in one channel to provide the correct phase relation so that the difference signal applied to speaker 162 is in fact the sum. The high frequencies, passed by capacitors C, are then reproduced by appropriately arranged speakers, connected to reproduce sum and difference signals with the inversion in the R f  speaker corrected by appropriate transposition of the voice coil connections.